Process for regenerating spent aqueous alkaline solution



Fatented Oct. 19 194 3- IJUNITED STA PROCESS FOR REGENERATQING SPENT AQUEOUS ALKALINE SOLUTION Lloyd Cfl etterly, Long Bea jcimibttli fipassignor to Shell Development Con'i Califi a corporationof Delaware A 7 No Drawin prises regeneration by air oxidation of aqueous c'austicalkali solutions which have been used to extract mercaptans from hydrocarbon distillates.

,It is known that mercaptans in aqueous alkali metal hydroxide solutions can be convert'edito neutral-insoluble disulfides by air oxidation in TApplication June 2 Serial No. 542,258

5 Claims. (Cl. 252-192) the presence of certain organic catalysts, thereby I producing a regenerated.treatedsolution. These catalysts are of the class which "undergo a hydro-"" quinone-quinone type reaction by oxidation and the reverse by reduction. Thus,.when the mercaptan content of the alkaline solution becomes low due to progressive oxidation, the catalyst, begins to oxidize in excess of the 'aboveinitial oxidation. Unfortunately, not all of the oxidation products of the catalyst are reducible again to the catalyst when the mercaptan content is increased. Therefore, in the past it'hasbeen the a practice to retain in the alkaline solution a sufiici'ent amount of mercaptans to protect the cata lyst against any oxidation. This; however, re-

sulted in difiiculties when it'was attempted to utilize the regenerated alkaline solution for further extracting mercaptans from hydrocarbons,

because some of these retained inrcaptans entered the hydrocarbons producing an unfavorable re-entry value. (Re-entrywalue is the amount of mercaptans reintroduced into a mercaptan-free hydrocarbon when contacted with a regenerated treating solution.) Even the most stable and active known catalysts, such as tannin, picramic acid, sulfonated indigo, etc., require protective amounts of mercaptan sulfur of not less than .1 and usually 2% or higher.

Now it has been discovered that mercaptans can be oxidized safely to much lower mercaptan sulfur contents, and even down to zero mercaptan sulfur content, provided a small amount of dis-;

solved inorganic sulfide is maintained in the solution.

Accordingly, it is a purpose of this invention to oxidize mercaptan'sin' alkaline aqueous solution and in the presence of organic catalysts soluble therein, to a lower mercaptan sulfur content than has heretofore been possible, without destruction of the catalyst. It is another purpose to regeneraterepeatedly by air oxidation alkaline aqueous solutions used inthe extraction of mercaptans pany, San: Francisco,

2 iror'ni sour hydrocarbon oils to produce regenerated'solutions' of'low re-entry values. Another purpose is .to carry out such oxidation in the presend Yof' a protective amount of dissolved inorganic sulfide sulfur, i. e. sulfide ions.

This invention comprises maintaining-during theoxidation 0f mercaptans in aqueous alkaline solution in the presence of dissolvedorganic catalyst to form neutral disulfidesan amount of dissolved inorganic sulfide sulfur which is suflicient to 'preventor tofininimize the oxidation'of said catalyst. i

I As mentioned before, suitable catalysts are capable'bff undergoing a reversible hydroquinonequinone type reaction; By this reaction is meant ail reversible oxidation-reduction of a compound whereinftwo orf more hydroxyl, amino and/or primary orlse'cori'dary radicals are attached either to two vicinal carbonatoms (1,2 positions) linked bya doublebond, or twoconjugated carbon atoms 2(1, fpositions)glinked'bytwo conjugated double b'orfds to produce a diketone or diimine respectively, thus; 1 T'OHOH oo ,-r-

|| u i l -e=o- .0.o-

oi .lTIHR NHR.I.. lfiIR .I|\|TR C+C ,The're areseveral types of compounds known which'are' capable of undergoing this reversible reaction, ,notably the ortho and para hydroxy pl'l enols, aminonphenols and amino anilines and ,h omologues. Another broad class is that of the water-soluble findigos including thioindigos. As a rule, purely aliphatic compounds are not suitable, R ill- 15118 above formulae represents an organicir adical. The 'c'arbcn atoms carryingthe oxygen or, nitrogen (as the case may be) should 'either bevmembers of an aromatic'nucleus or else I should forma chainbetween two aromatic nuclei. Sincethe eatalystsshould be soluble in aqueous caustic alkali; the. presence of a strong acid radical, such as carboxyl, sulfonic, phosphonic,-:etc., radical, is desirable although it is not essential. compounds of; the phenol and aniline type the presence o f ameta directing radical is also highly desira ble in order testabilize the nucleus and-t0 lrmi'nimifze undesirable side reactions .lresultir g v in Catalysts are usually employed in concentr-altions varying from about .05% to and'pref erably from about .2% to 2%.

The minimum amount of sulfur required to protect the. catalyst varies considerably with, the nature and concentration of ,the catalyst, as welLas Lwith the amount of residual mercaptan sulfur which is allowed in the solution at thepointof completion of the oxidation. The; lower: the final mercaptin sulfur content the more sulfide sulfur is required for lasting protection of the catalyst. In the complete absence of mercaptan sulfur, it is desirable to main- -or 4-amino as, say, 5%.

-benzoic acids, -sulfonyl;

sultonated inand indigo white inorganic sulfide tain-between about .2% and 1% sulfur in the form of dissolved inorganic sulfide, although lesser amounts, say 1%, may give reasonable protection to the catalyst.

The presence of mercaptan sulfur at the end of the oxidation usually allows a proportionally,-

3 lower minimum sulfide sulfur content. Mercaptans, as a rule, oxidize more readily than inorganic sulfides. Therefore, it is usually possible to reduce the mercaptan sulfur content to very .low levels without materially oxidizing the suln fides. However, if the mercaptan sulfur content is reduced to zero, at least a portion'of the sulfide content may oxidize as well. i. a

The practical significance of the above is this:

heretofore, it was difficult, if not impossible, to;

reduce the mercaptan sulfur content to below .l% to .2% without also oxidizing the catalyst non-reversibly Many catalysts require minimum protective amounts of mercaptan sulfur considerablyv above this limit. At these mercaptanv sulfur levels the re-entry 'valueis quite high,

usually too high to produce doctor-sweet hydrocarbon distillate. However, in the presence of from .1 to l% dissolved inorganicsulfide sulfur, the merc-aptan sulfur content can be reduced to.

a lower limit without materially damaging the" catalyst, thereby improving the re-entry value of f the regenerated solution.

As indicated above, as the mercaptan sulfur content is lowered by oxidation to below about 1 3%, some of the sulfide may begin to oxidize as Well. The oxidation products of the sulfides comprise free sulfur and may include thiosulfates, polythionates and sulfates. Free sulfur, if formed, may be removed by filtration. Thiosulfates,. thionates and sulfates, etc., up, reduce the alkalinity of the aqueous treating solution thereby making it less effective for mercaptan extraction, although a small amount of thiosulfates, for example, may be helpful in protecting the catalyst. These salts may also cause" tion of the inorganic sulfides at the lowest pracif allowed to build solution, or unless all of the solution is discarded when loaded with neutral salts. By the same token, it may be desirable not to maintain too high a concentration of dissolved sulfide sulfur, although it may be permissible to have it as high If the sulfide sulfur content is between thepreferred limits indicated earlier and the residual mercaptan sulfur is closely controlled, the rate at which neutral salts will build up can beso slow that their content will never be undesirably high. This is the case particularly if a catalyst is used which is highly selective for the oxidation of mercaptan sulfur as against sulfide sulfur, such as the poly hydroxy or amino hydroxy benzenes having at least one meta directing substitution radical in the nucleus.

Portions of the sulfide sulfur which are oxidized should be replaced to prevent dropping of the sulfide sulfur level to below the minimum protective limit. Replacement maybe accomplished in any desired way, as for example .by the addition of H28. I

In practice, where themercaptide solutions are normally obtained'in the treatment of sour hydrocarbon distillates,v containing I-IzS and mercaptans, with aqueous alkaline solutions, I-lzS'Will enter the treating solution unless it has been removed from the distillate. In order to avoid accumulation of] excessive quantities of sulfides in the treating solution, it may be desirable to pre-remove a portion of the 1-128 from the distillate.

The aqueous caustic alkaline solution may, if desired,'containsubstantial amounts of solubility promotersfor'mercaptans such as water-soluble mono orpolyl'iydricalcohols; ethers or esters; alkynolamines or aliphatic ,diamines; or fatty acidshaving from about 2 to 6 carbon atoms; naphthenic acids and/or alkyl phenols, etc.

The following examples illustrate the invention:

xam I V A spent sodium hydroxide solution obtained in the treatment of hydrocarbon distillates and containing mercaptides and sodium sulfide was oxidized by air blowing in the presence of a .2% tannic. acidtozero mercaptan content. Mercaptans were added repeatedly and blowing was continued each time to zero mercaptan sulfur content. The sulfide'content wasreduced slowly.

A spent sodium hydroxide solution similar to that of Examplel was.,oxidized'by air blowing in the presenceof severalcatalysts. As will be noted,

"the selectivity for preferentially oxidizing mertical value unless a portion of the solution is I continuously withdrawn and replaced with fresh captan sulfur as against sulfide sulfur varied considerablywith different catalysts.

Azr blowing Total Tempera- Sulfide Mercaptan catalyst Hours ture, F. Sulfur Sulfur .2 Quebracho 2 122 53 32 D 3 122 53 08 Do 4 122 51 0. 0 1% Pyrogall 1, 2 122 48 .18 D0 2 122 40 0. 0 DO 2 80 47 29 D0 3 80 46 16 D0 4 80 45 07 D0 80 44 0. 0 1% Picramic Aci 2 70 .54 0.0 .1% Indigo Sulfonic Acid l 80 50 0. 0

EXAMPLE III A spent sodium hydroxide solution similar to that of Example I containing .67% by wt. of NaOH, .l% by wt. of a phenolic catalyst, .60% by wt. sulfide sulfur, and .77% by wt. of mercaptan sulfur was air-blown for 5 hrs. at 80 F. to a zero mercaptan sulfur content and to produce 33% by wt. free sulfur. This blown solution was then respent with light mercaptans to .98 by wt. of mercaptan sulfur. The respent solution after airblowing for 3 hrs. contained 25% of sulfide sulfur and no mercaptan sulfur. The catalyst was still active.

In comparison, a similar spent caustic solution containing by wt. 2.5% NaOH, .05% phenolic catalyst, 1.25% mercaptan sulfur and no sulfide sulfur was air-blown for 5 hrs. with oxidation of the mercaptan sulfur. The catalyst, howevehad lost most of its activity.

I claim as my invention:

1. In a process for regenerating spent aqueous alkaline solution containing mercaptans extracted from hydrocarbons, the improvement comprising treating said spent solution with a free oxygen-containing gas in the presence of a phenolic oxidation catalyst, thereby oxidizing said mercaptans to organic disulfides which catalyst is capable of a reversible oxidation reaction and subject to non-reversible oxidation by oxygen in an alkaline solution in the absence of a protecting reducing agent, and maintaining dissolved in said solution during said oxidation treatment, by extraneous addition, an inorganic sulfide sulfur content of at least 0.1% by weight of sulfide sulfur suificient to protect said catalyst from non-reversible oxidation.

2. In a process for regenerating spent aqueous alkaline solution containing mercaptans extracted from hydrocarbons, the improvement comprising treating said spent solution with a free oxygen-containing gas in the presence of a phenolic oxidation catalyst, thereby oxidizing said mercaptans to organic disulfides, which catalyst is capable of a reversible oxidation reaction and subject to non-reversible oxidation by oxygen in an alkaline solution in the absence of a protecting reducing agent, and maintaining dissolved in said solution during said oxidation treatment, by extraneous addition, an alkali metal sulfide sulfur content of from about 0.2% to about 1% by weight of sulfide sulfur whereby non-reversible oxidation of said catalyst is minimized.

3. In a process for regenerating spent aqueous alkaline solution containing mercaptans extracted from hydrocarbons, the improvement comprising treating said spent solution with a free oxygen-containing gas in the presence of tannin under oxidizing conditions, thereby oxidizing said mercaptans to organic disulfides, and maintaining dissolved in said solution during said oxidation treatment, by extraneous addition, an inorganic sulfide sulfur content of at least 0.1% by weight of sulfide sulfur sufficient to protect the tannin from non-reversible oxidation.

4. In a process for regenerating spent aqueous alkaline solution containin mercaptans extracted from hydrocarbons, the improvement comprising treating said spent solution with a free oxygen-containing gas in the presence of picramic acid under oxidizing conditions, thereby oxidizing said mercaptans to organic disulfides, and maintaining dissolved in said solution during said oxidation treatment, by extraneous addition, an inorganic sulfide sulfur content of at least 0.1% by weight of sulfide sulfur sufiicient to protect the picramic acid from non-reversible oxidation.

5. In a process for regenerating spent aqueous alkaline solution containing mercaptans extracted from hydrocarbons, the improvement comprising treating said spent solution with a free oxygen-containing gas in the presence of pyrogallol under oxidizing conditions, thereby OXidiZillg said mercaptans to organic disulfides, and maintaining dissolved in said solution during said oxidation treatment, by extraneous addition, an inorganic sulfide sulfur content of at least 0.1% by weight of sulfide sulfur sufficient to protect the pyrogallol from non-reversible oxidation.

LLOYD C. FE'I'TERLY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

